Hybrid–PIC simulation of sputtering product distribution in a Hall thruster

Hall thrusters have been widely used in orbit correction and the station-keeping of geostationary satellites due to their high specific impulse,long life,and high reliability.During the operating life of a Hall thruster,high-energy ions will bombard the discharge channel and cause serious erosion.As time passes,this sputtering process will change the macroscopic surface morphology of the discharge channel,especially near the exit,thus affecting the performance of the thruster.Therefore,it is necessary to carry out research on the motion of the sputtering products and erosion process of the discharge wall.To better understand the moving characteristics of sputtering products,based on the hybrid particle-in-cell (PIC) numerical method,this paper simulates the different erosion states of the thruster discharge channel in different moments and analyzes the moving process of different particles,such as B atoms and B+ ions.In this paper,the main conclusion is that B atoms are mainly produced on both sides of the channel exit,and B+ ions are mainly produced in the middle of the channel exit.The ionization rate of B atoms is approximately 1％.     

Numerical study of low-frequency discharge oscillations in a 5kW Hall thruster

A two-dimensional particle-in-cell plasma model is built in the R–Z plane to investigate the lowfrequency plasma oscillations in the discharge channel of a 5 kW LHT-140 Hall thruster. In addition to the elastic, excitation, and ionization collisions between neutral atoms and electrons,the Coulomb collisions between electrons and electrons and between electrons and ions are analyzed. The sheath characteristic distortion is also corrected. Simulation results indicate the capability of the built model to reproduce the low-frequency oscillation with high accuracy. The oscillations of the discharge current and ion density produced by the model are consistent with the existing conclusions. The model predicts a frequency that is consistent with that calculated by the zero-dimensional theoretical model.     

Simulation of the effect of a magnetically insulated anode on a low-power cylindrical Hall thruster

The intersection point of the characteristic magnetic field line (CMFL) crossing the anode boundary with the discharge channel wall,and its influence on thruster performance and the energy and flux of ions bombarding the channel wall,have been studied numerically.The simulation results demonstrate that with the increase in distance from the crossover point of the CMFL with the channel wall to the bottom of the thruster channel,the ionization rate in the discharge channel gradually increases;meanwhile,the ion energy and ion current density bombarding the channel wall decreases.When the point of the CMFL with the channel wall is at the channel outlet,the thrust,specific impulse,and efficiency are at a maximum,while the ion energy and ion current density bombarding the channel wall are at a minimum.Therefore,to improve the performance and lifetime of the thruster,it is important to control the point of intersection of the CMFL with the channel wall.     

Study on the influences of ionization region material arrangement on Hall thruster channel discharge characteristics

There exists strong interaction between the plasma and channel wall in the Hall thruster,which greatly affects the discharge performance of the thruster.In this paper,a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel.The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel.The influences of segmented electrode placed at the ionization region on electric potential,ion number density,electron temperature,ionization rate,discharge current and specific impulse are discussed.The results show that,when segmented electrode is placed at the ionization region,the axial length of the acceleration region is shortened,the equipotential lines tend to be vertical with wall at the acceleration region,thus radial velocity of ions is reduced along with the wall corrosion.The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region.Furthermore,the electron-wall collision frequency and ionization rate also increase,the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.     

Study on the characteristics of non-Maxwellian magnetized sheath in Hall thruster acceleration region

The secondary electron emission (SEE) and inclined magnetic field are typical features at the channel wall of the Hall thruster acceleration region (AR), and the characteristics of the magnetized sheath have a significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this work, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with SEE in the AR of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized. Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameter and magnetic field on the AR sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly, the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion. Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the $x$ direction acting as a resistance decelerating ions becomes larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential, secondary electron number density and sheath thickness.     

Particle-in-cell simulation for effect of anode temperature on discharge characteristics of a Hall effect thruster

Propellant gas flow has an important impact on the ionization and acceleration process of Hall effect thrusters (HETs). In this paper, a particle-in-cell numerical method is used to study the effect of the anode temperature, i.e., the flow speed of the propellant gas, on the discharge characteristics of a HET. The simulation results show that, no matter the magnitude of the discharge voltage, the calculated variation trends of performance parameters with the anode temperature are in good agreement with the experimental ones presented in the literature. Further mechanism analysis indicates that the magnitude of the electron temperature is responsible for the two opposing variation laws found under different discharge voltages. When the discharge voltage is low, the electron temperature is low, and so is the intensity of the propellant ionization; the variation of the thruster performance with the anode temperature is thereby determined by the variation of the neutral density that affects the propellant utilization efficiency. When the discharge voltage is high, the electron temperature is large enough to guarantee a high degree of the propellant utilization no matter the magnitude of the anode temperature. The change of the thruster performance with the anode temperature is thus dominated by the change of the electron temperature and consequently the electron-neutral collisions as well as the electron cross-field mobility that affect the current utilization efficiency.     

Matching characteristics of magnetic field configuration and chamfered channel wall in a magnetically shielded Hall thruster

To date,the selection of the magnetic field line used to match the chamfered inner and outer channel walls in a magnetically shielded Hall thruster has not been quantitatively studied.Hence,an experimental study was conducted on a 1.35 kW magnetically shielded Hall thruster with a xenon propellant.Different magnetic field lines were chosen,and corresponding tangentially matched channel walls were manufactured and utilized.The results demonstrate that high performance and a qualified anti-sputtering effect cannot be achieved simultaneously.When the magnetic field lines that match the chamfered wall have a strength at the channel centerline of less than 12％ of the maximum field strength,the channel wall can be adequately protected from ion sputtering.When the magnetic field lines have a strength ratio of 12％-20％,the thruster performance is high.These findings provide the first significant quantitative design reference for the match between the magnetic field line and chamfered channel wall in magnetically shielded Hall thrusters.     

The far-field plasma characteristics of LHT40 low-power Hall thruster for commercial aerospace applications

To achieve a better insight into the far-field plasma spatial distribution and evolution characteristics of the 300 W class low-power Hall thruster (LHT) for commercial aerospace applications,a dedicated and integrated plasma diagnostic system composed of seventeen Faraday cups (FC) and two triple Langmuir probes (TLP) is established to investigate the time-averaged in situ spatial distribution characteristics of far-field ions and electrons.The ion current density (ICD),plasma potential,plasma density,and electron temperature at 1000 mm downstream of 300 W class LHT for commercial aerospace applications in the azimuthal angle range of-90° to 90° were investigated under the conditions of different anode mass flow rates and discharge voltages.The results demonstrated that ICD,beam divergence angle,and mass utilization efficiency increased with increasing anode mass rate.The double-wings phenomenon was observed in the spatial distribution of ICD at large angles from the thruster axis,which is attributed to charge exchange collisions at increasing vacuum backpressure.The plasma electron temperature,electron density,and plasma potential parameters derived from the TLP decreased rapidly in the angle range from 0° to 30° and did not exhibit significant variations above 30°,which was also in good agreement with the results of the measured divergence angle of the FC.The discrepancy of average ion speed was calculated.The maximum error is better than 31.5％which checks the consistency between the TLP's results and that of FC to some extent.     

A method to measure the in situ magnetic field in a Hall thruster based on the Faraday rotation effect

This paper presents a method to measure the in situ magnetic field in a Hall thruster by optical non-invasive means, based on the optical Faraday rotation effect. This method does not affect the discharge of the thruster. Furthermore, its time resolution depends on the speed of the photodetector, and measurement at a MHz scale can be achieved.     

Numerical study of the effect of aft-loaded magnetic field on multiple ionizations in Hall thruster

It is assumed that the shift of a strong magnetic field region with a positive gradient from exit plane to outside, namely the transit from a normal loaded magnetic field to an aft-loaded one, enhances the multiple ionization process in the magnetically shielded Hall thruster. To confirm this conjecture, a comparative study is carried out numerically with a particle-in-cell method. The simulation results prove that compared with the normal loaded magnetic field, the application of aft-loaded magnetic field enhances the multiple ionization process. This study further analyzes the ionization characteristics of the transition from low-charged ions to high-charged ions under two magnetic field conditions and the influence of the magnetic strength of aft-loaded magnetic field on the multiple ionization characteristics. The study described herein is useful for understanding the discharge characteristics of Hall thruster with an aft-loaded magnetic field.     

The effect of anode axial position on the performance of a miniaturized cylindrical Hall thruster with a cusp-type magnetic field

A 200 W cylindrical Hall thruster with a cusp-type magnetic field was proposed, manifesting convergent plume and high specific impulse. In this paper, a series of ring-shaped anodes are designed and the influence of anode axial position on the performance of CHT with a cusp-type magnetic field is studied. The experimental results indicate that the thruster keeps stable operation at the condition of 140–270 W discharge power. When the anode moves axially towards the upstream cusp field, the thrust enhances from 6.5 mN to 7.6 mN and specific impulse enhances from 1658 s to 1939 s significantly. These improvements of thruster performance should be attributed to the enhancement of current utilization, propellant utilization and acceleration efficiency. According to the analyses on the discharge characteristics, it is revealed that as the anode moves upstream, the electron transport path could be extended, the magnetic field in this extended path could impede electron cross-field transport and facilitate the ionization intensity, yielding to the enhancement of current utilization and propellant utilization efficiency. Moreover, along with this enhancement of upstream ionization at the given anode flow rate, the main ionization region is thought to move upstream and then separate more apparently from the acceleration region, which has been demonstrated by the narrowing of ion energy distribution function shape. This change in acceleration region could decrease the ion energy loss and enhance acceleration efficiency. This work is beneficial for optimizing the electrode structure of thruster and recognize the ionization and acceleration process under the cusp magnetic field.     

Simulation study on the influence of magnetic field in the near-anode region on anode power deposition of ATON-type Hall thruster

The highest deposition of power and temperature is always near the cusp of the ATON-type Hall thruster. This shows that when there are electrons gathering at the cusp, the distribution of heat load will be uniform, which will potentially damage the reliability. Therefore, we optimize the magnetic field near the anode. We changed the magnetic field characteristics in the near-anode region with an additional magnetic screen, and performed numerical simulation with particle-incell simulation. The simulation results show that the magnetic field of the thruster with the additional magnetic screen can alleviate the over-concentration of power deposition on the anode and reduce the power deposition in the anode by 20%, while ensuring that the overall magnetic field characteristics do not change significantly.     

Machine learning-based method to adjust electron anomalous conductivity profile to experimentally measured operating parameters of Hall thruster

The problem of determining the electron anomalous conductivity profile in a Hall thruster, when its operating parameters are known from the experiment, is considered. To solve the problem, we propose varying the parametrically set anomalous conductivity profile until the calculated operating parameters match the experimentally measured ones in the best way. The axial 1D3V hybrid model was used to calculate the operating parameters with parametrically set conductivity. Variation of the conductivity profile was performed using Bayesian optimization with a Gaussian process (machine learning method), which can resolve all local minima, even for noisy functions. The calculated solution corresponding to the measured operating parameters of a Hall thruster in the best way proved to be unique for the studied operating modes of KM-88. The local plasma parameters were calculated and compared to the measured ones for four different operating modes. The results show the qualitative agreement. An agreement between calculated and measured local parameters can be improved with a more accurate model of plasma-wall interaction.     

Numerical research of a 2D axial symmetry hybrid model for the radio-frequency ion thruster

Since the high efficiency discharge is critical to the radio-frequency ion thruster (RIT),a 2D axial symmetry hybrid model has been developed to study the plasma evolution of RIT.The fluid method and the drift energy correction of the electron energy distribution function (EEDF) are applied to the analysis of the RIT discharge.In the meantime,the PIC-MCC method is used to investigate the ion beam current extraction character for the plasma plume region.The beam current simulation results,with the hybrid model,agree well with the experimental results,and the error is lower than 11％,which shows the validity of the model.The further study shows there is an optimal ratio for the radio-frequency (RF) power and the beam current extraction power under the fixed RIT configuration.And the beam extraction efficiency will decrease when the discharge efficiency beyond a certain threshold (about 87 W).As the input parameters of the hybrid model are all the design values,it can be directly used to the optimum design for other kinds of RITs and radio-frequency ion sources.